1. Field of the Invention
This invention relates to the well-known continuous cyclic fluid catalytic cracking (FCC) of hydrocarbons with a catalyst, generally a catalyst containing a crystalline aluminosilicate zeolite component, in the absence of added hydrogen to produce gasoline, which cracking results in the formation on the catalyst particles of a deposit of combustible hydrocarbons known as coke, and the spent catalyst particles from the catalytic reactor are regenerated in a separate zone by burning off sufficient coke to place the catalyst particles in a condition suitable for recycling to the hydrocarbon conversion zone. In particular the invention is concerned with a solid additive capable of promoting combustion of carbon monoxide to carbon dioxide in FCC regenerators without appreciably affecting the ability of the catalyst particles to catalyze the hydrocarbon conversion reaction in the conversion cycle.
Present-day continuous cyclic FCC processes utilize fluidizable catalyst particles containing a crystalline zeolitic aluminosilicate component (usually an ion-exchanged form of a synthetic faujasite such as zeolite X or Y) and a porous inorganic oxide matrix. This type of catalyst must be regenerated to low carbon levels, typically 0.5% or less, to assure required activity and selectivity before the catalyst particles can be recycled to a conversion zone. In most regenerators, the combustible solids deposited on the spent solid catalyst particles from the cracking zone are burned in a confined regeneration zone in the form of a fluidized bed which has a relatively high concentration of catalyst particles (dense phase). A region of lower solids concentration (light phase) is maintained above the dense phase. A typical regeneration cycle is described in U.S. Pat. No. 3,944,482 to Mitchell.
High residual concentrations of carbon monoxide in flue gases from regenerators have been a problem since the inception of catalytic cracking processes. The evolution of FCC has resulted in the use of increasingly high temperatures in FCC regenerators in order to achieve the required low carbon levels in the regenerated crystalline aluminosilicate catalysts. Typically regenerators now operate at temperatures in the range of 1100 to 1350.degree. F. and result in flue gases having a CO.sub.2 /CO ratio in the range of 1.5 to 0.8. The oxidation of carbon monoxide is highly exothermic and can result in so-called "carbon monoxide afterburning" which can take place in the dilute catalyst phase, in the cyclones or in the flue gas lines. Afterburning has caused significant damage to plant equipment. On the other hand, unburned carbon monoxide in atmosphere-vented flue gases represents a loss of fuel value and is ecologically undesirable.
Restrictions on the amount of carbon monoxide which can be exhausted into the atmosphere and the need for efficient coke removal from spent catalyst particles have stimulated several approaches to the provision of means for achieving a balance between afterburning and incomplete regeneration of spent fluid zeolitic catalysts.
It is well known that metals such as iron, nickel, vanadium and copper can promote carbon monoxide when present as contaminants in cracking feedstocks. Early in the development of catalytic cracking and long prior to the introduction of crystalline zeolite aluminosilicate catalysts, it was proposed (U.S. Pat. No. 2,436,927 to Kassel) to prevent afterburning in fluidized catalytic cracking processes by introducing a small amount of a carbon monoxide oxidizing catalyst. The proposed oxidant was an oxide of metals from the first transition series. It was suggested to introduce such material either as a component of the cracking catalyst or, preferably, as separate particles supported "on a suitable carrier". Such carrier was not described in the patent. Chromium oxide was proposed as an impregnant for gel-type moving bed cracking catalysts in U.S. Pat. No. 2,647,860 to Plank et al. This was also prior to the introduction of crystalline zeolitic catalysts. Subsequently it was suggested to incorporate titanium in cracking catalysts for improved carbon monoxide conversion but this approach was directed to achieve only partial combustion of carbon monoxide since regenerators available at that time were not capable of withstanding the heat release resulting from full combustion.
U.S. Pat. No. 3,364,136 to Chen suggested the use of a noble metal such as platinum to promote carbon monoxide oxidation in a regenerator of a FCC unit operated with a zeolitic aluminosilicate catalyst. According to the teachings of the patent, the noble metal had to be held within the inner pore structure of a so-called "shape selective" zeolite, specifically a zeolite having pores large enough to allow penetration of oxygen, carbon monoxide and carbon dioxide but too small for molecules of gas-oil. In one preferred embodiment, the particles of shape selective zeolite containing the oxygen promoter within the pores were contained in the same particles which included both the larger pore catalytically active zeolite and a conventional inorganic oxide matrix component. For example, the two different zeolites, one including a promoter such as platinum within the pores, were composited into unitary particles with an inorganic oxide matrix material. An alternative disclosed in the Chen patent involved mixing the particles of sieve containing the oxidation promoter with particles of the zeolitic catalyst. In a preferred embodiment of this alternative, the individual components were of different particle size so that the oxidation component could be withdrawn as well as added to the circulating catalyst mass to alter the degree of carbon monoxide conversion. In all variations of this technology, preparation of a costly small pore zeolitic component is required and the oxidant will be present on a high surface area support.
According to the teachings of West German Application DT No. 2444911, small amounts of metal or metallic elements of Period 5 and 6 of Group VIII of the Periodic Table or rhenium or compounds thereof are simply added in amounts up to 50 p.p.m. to conventional FCC (or TCC) catalysts to decrease the carbon monoxide content of flue gases, as evidenced by the improved CO.sub.2 /CO ratio of such gases, without appreciably affecting the cracking properties of the catalysts. The metal component, preferably a platinum compound, is introduced into the catalyst by impregnation or by ion exchange during any stage of catalyst manufacture, or even after the catalyst particles are formed. According to the teachings of the German patent application, the active cracking catalyst component (zeolitic aluminosilicate) is preferably ion-exchanged with the metal and the ion-exchanged material is composited with the porous matrix to produce catalyst particles. The German application also discloses that a silicon-containing support or clay can be ion-exchanged or impregnated with the metal but there is no explanation as to how this is accomplished. Based on illustrative examples, a reasonable interpretation is that the exchanged support or clay is mixed with the catalytically active zeolite component to form composite catalyst particles in which the metal promoter and active zeolite are present in the same particles.
The patented techniques for preparing a platinum metal promoted cracking catalyst leave something to be desired. Impregnation or ion-exchange of the zeolite or the porous matrix before compositing the constituents can be used only in the production of those catalysts in which the zeolite is formed separately from the matrix; for example, catalysts prepared as described in U.S. Pat. Nos. 3,140,249 and 3,140,253 to Plank et al. When a finished catalyst is treated, the entire tonnage of catalyst must be processed. Similarly, the entire catalyst must be treated with a metal when the catalyst particles are produced in situ from preforms, such as catalysts produced in accordance with the teachings of U.S. Pat. No. 3,647,718 to Haden et al. By way of example, in Example 10 of the DT No. 2444911 application, a promoted FCC catalyst was prepared containing 3 p.p.m. platinum by impregnating clay-based catalyst with a solution of platinum-tris (ethylenediamine) tetrachloride followed by washing and drying. Using this technique on a commercial basis, the production of 10,000 tons of metal-promoted catalyst would require the use of about 35,000 tons of platinum solution to incorporate the desired 3 p.p.m. platinum. This would necessitate a substantial capital investment for equipment for impregnation, washing and drying. Prior to our invention, the suggestion was made that the platinum oxidation might be incorporated on a solid support material. Presumably, a conventional high surface area gel-type catalyst was intended as the support.
A general object of the invention is to provide improvements in prior art means for achieving controlled oxidation of carbon monoxide in the regeneration zone of a cyclic FCC process.